This invention relates to a method of assembling an instrument and more particularly to a resistance temperature detector wherein assembly requires insertion of a relatively delicate element having an attached electrical lead into a receiving cavity. Specifically, the invention relates to assembly of a resistance temperature detector such as that described in copending U.S. patent application Ser. No. 581,426 filed on Feb. 17, 1984 titled Temperature Probe [hereinafter Temperature Probe specification].
Resistance thermometers or temperature detectors have been used in many industrial applications. A resistance thermometer depends upon the variation of the electrical resistance of a wire with temperature. Most metals become more resistant to the passage of an electrical current as the metal increases in temperature. The increase in resistance is generally proportional to the rise in temperature. Thus, a constant current passed through a metal of varying resistance produces a variation in voltage that is proportional to the temperature change.
In many cases, resistance temperature detectors and similar instruments must be designed and constructed to be shock resistant. For example, many resistance temperature detectors employ a sensing element in the form of a platinum wire having a very small diameter. The platinum wire is wound around a ceramic or glass mandrel or other support to form a sensing element. The wire is secured by coating the wire and the mandrel or support with a refractory cement or glass.
The sensing element is placed in contact with a signal conditioner by use of lead wires, which are fused to the sensing element. The sensing element is then inserted inside a piece of hollow cylindrical metal tubing or casing, which is normally made of stainless steel. The sensing element is then secured. For example the space between the interior walls of the hollow cylindrical metal tubing and the sensing element may be filled with a compacted refractory powder such as magnesium oxide or a ceramic cement to prevent damage to the sensor due to vibration and otherwise secure the sensing element.
The patent literature discloses a variety of temperature measuring devices. MacKenzie et al, U.S. Pat. No. 4,087,775 is illustrative here. According to that disclosure, there is provided a temperature measuring probe and a mode of assembling the element in a reliable manner against shock and vibration. More particularly, according to that disclosure, there is provided a commercial resistance temperature detector element including a solid insulating mandrel upon which a helical winding of suitable resistance wire is wound. The winding is encased in glass fused in place or fired ceramic and the terminal ends of each winding are brought out for connection to a suitable circuit. The resistance temperature detector is supported in a metallic sheath made of stainless steel. The element is supported in and insulated with respect to the sheath by means of a firmly compacted mass of a pulverized or refractory material such as magnesium oxide. The terminal leads of the element and the pulverized refractory material are leveled adjacent one end of the sheath, as by counterboring, to provide a chamber to house the connections between the terminal leads of the resistance temperature detector element as conductors. The resulting chamber has an interior portion of the sheath as its lateral wall. A bore is then drilled on the principal axis of the sheath of such length and diameter as to receive the sensing element with a comfortable fit yet loose enough to allow placement of an adequate layer of cement. After insertion of the element into the bore and curing of the ceramic cement, conductors are welded to the terminal leads to connect the resistance temperature detector element with the appropriate measuring devices.
Chambers, U.S. Pat. No. 3,267,733 is further illustrative of resistance thermometers disclosed in the patent literature. According to that disclosure, an elongated resistive element in the form of an interwound spiral is encapsulated and supported in an oxide encapsulation. The oxide encapsulation is supported with a refractive metal or ceramic tube, which in turn is suitably secured to a sleeve. Wires connecting to the elongated resistive element pass through the sleeve 12, where they are supported by a pure beryllium oxide insulation.
Beaudoin et al, U.S. Pat. No. 4,011,654 discloses an exhaust gas sensor probe. According to that disclosure, a tube of ceramic material is arranged to support a resistive type ceramic sensor. The tube is provided with a generally continuous groove around the portion of its exterior to support a heater wire in close heat conductive relation to the sensor. A housing sleeve is provided with a plurality of fin members. The fin members provide for radiation of heat energy to protect the electrical terminals while providing a convenient mechanism to control the depth of penetration of the probe within an associated exhaust system so that the sensor will be properly positioned at the approximate center of the associated exhaust gas conduit.
The following patents are further illustrative of other temperature sensing devices: Picciano, U.S. Pat. No. 2,379,317; and Micheli et al, U.S. Pat. No. 3,896,409.
These and other prior temperature sensing devices and temperature probes suffer from one or more defects or limitations. Many of the probes are too fragile in construction to provide sufficient ruggedness. For example, the device may fail or, after several months exposure to a process stream, suffer an unacceptable reduction in response time.
Still other probes are designed such that probe seals have a reduced life expectancy, thus requiring more expensive seals or their frequent replacement. Further, a failure of the seal may lead to low insulation resistance from moisture penetration and a loss of accuracy. The failure of the seals is often caused by conduction of heat from the temperature probe. Such heat conduction can also cause failure of wire insulation, thus leading to undesirable results.
These and other limitations of prior temperature sensing devices are minimized by the device disclosed in the above-referenced Temperature Probe specification.
In one embodiment of the resistance temperature detector disclosed in the above-referenced copending patent application, a temperature probe is provided for measuring the temperature of the probe's surroundings. A metallic tip casing is provided with at least one receiving cavity. A sensor is mounted into each receiving cavity and surrounded by packing. Each sensor is made of a substrate and a temperature sensing element mounted on the substrate. An electrical lead is also provided for placing the sensor in electrical communication with a signal conditioner.
The potentially small dimensions of the receiving cavities and the sensors, and the difficulties in manipulating multiple sensors with their electrical leads during assembly, make it desirable to have a method that facilitates assembly of a detector. Prior methods of assembly suffer from one or more of several defects or limitations. For example, the relative dimensions or overall design of the probes are limited at least in part by the need to maintain quality control during assembly. Other methods are time consuming and require relatively extensive use of additional equipment during assembly. These and other limitations of prior assembly methods are minimized by the present inventive method.